1) Field of Invention
This invention relates generally to apparatus and methods for conveying energy into a volume of fluid and more specifically to the field of linear pumps, linear compressors, synthetic jets, resonant acoustic systems and other fluidic devices.
2) Description of Related Art
For the purpose of conveying energy to fluids within a defined enclosure, prior technologies have employed a number of approaches, including positive displacement, agitation such as with mechanical stirring or the application of traveling or standing acoustic waves, the application of centrifugal forces and the addition of thermal energy. The transfer of mechanical energy to fluids by means of these various methods can be for a variety of applications, which could include for example, compressing, pumping, mixing, atomization, synthetic jets, fluid metering, sampling, air sampling for bio-warfare agents, ink jets, filtration, driving physical changes due to chemical reactions, or other material changes in suspended particulates such as comminution or agglomeration, or a combination of any of these processes, to name a few.
Within the category of positive displacement machines, diaphragms have found widespread use. The absence of frictional energy losses makes diaphragms especially useful in downsizing positive displacement machines while trying to maintain high energy efficiency. The interest in MESO and MEMS scale devices has lead to even further reliance on diaphragm type and diaphragm/piston (i.e. a piston with a flexible surround) type devices for conveying energy into fluids within small pumps or other fluidic devices. The term “pump” as used herein refers to devices designed for providing compression and/or flow for either liquids or gases. The term “fluid” used herein is understood to include both the liquid and the gaseous states of matter.
The actuators used to drive larger diaphragm pumps have proved problematic for MESO or MEMS machines since it is difficult to maintain their efficiency and low cost as they are scaled down in size. For example, the air gaps associated with electromagnetic and voice coil type actuators must be scaled down in order to maintain high transduction efficiency and this adds manufacturing complexity and cost. Also, motor laminations become magnetically saturated as motors are scaled down while seeking to maintain a constant mechanical power output. Within acceptable product cost targets, it is widely accepted that the electro-mechanical efficiency of these transducers will drop off significantly with size reduction.
These scaling challenges, associated with conventional magnetic actuators, have led to the widespread use of other technologies, such as electrostrictive actuators (e.g. piezoceramics), piezoceramic benders, electro-static and magnetostrictive actuators for MESO and MEMS applications. A piezo bender disk can naturally combine the fluid diaphragm and actuator into a single component.
The advantages of using the piezo as the fluidic diaphragm are offset by the piezo's inherent displacement limitations. Since ceramics are relatively brittle, piezoceramic diaphragms/disks can only provide a small fraction of the displacements provided by other materials such as metals, plastics, and elastomers, for example. The peak oscillatory displacements that a clamped circular piezoceramic disk can provide without failure are typically less than 1% of the disk's clamped diameter. Since diaphragm displacement is directly related to the fluidic energy transferred per stroke, piezo benders impose a significant limitation on the power density and overall performance of small fluidic devices such as MESO-sized pumps and compressors. These displacement-related energy limitations are especially true for gases.
Other types of piezo actuators that depend on the bulk flexing properties of the piezo material can provide high energy transfer to liquids by operating at very high frequencies, but at even smaller strokes. These small actuator strokes make the design of pumps impractical. Further, high-performance pumps employ passive valves that open and close each pumping cycle to provide optimal pumping efficiency. These pump valves may not provide the needed performance in the kHz-MHz frequency range that bulk-piezo actuators need to transfer sufficient energy.
Currently, the demand is increasing for ever smaller fluidic devices which may not be attainable or functionally consistently useful with current piezo pump technology. For example, pumps and compressors are needed that can provide higher power densities and specific flow rates (i.e. fluid volume flow rate divided by the pump's physical volume) at higher pressure heads and in ever smaller sized units. Examples of applications that require high performance MESO-sized pumps include the miniaturization of fuel cells for portable electronic devices such as portable computing devices, PDAs and cell phones; self-contained thermal management systems that can fit on a circuit card and provide cooling for microprocessors and other semi-conductor electronics and portable personal medical devices for ambulatory patients. Thus, there is a need for a compact economically viable piezo pump that remedies at least some of the deficiencies of current piezo pumps.